Catalyst carrier holding mat and process for production of catalytic converter

ABSTRACT

A catalyst carrier holding mat is rolled around a catalyst carrier so as not to wrinkle when rolled, and the catalyst carrier is thereby uniformly compressed.  
     Furthermore, method of forming a catalytic converter in this manner is also provided.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to a mat which holds a catalyst carrier of a catalytic converter which cleans exhaust gas exhausted from an engine of cars or the like in a casing by being rolled around the catalyst carrier, and in particular, relates to a process for production of the catalytic converter.

[0003] 2. Background Art

[0004] Generally, a three-way catalytic converter is arranged in part of an exhaust duct and is used in a method of cleaning exhaust gas. This three-way catalytic converter changes three noxious ingredients contained in exhaust gas such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO_(X)) into harmless substances in the entirety by a catalyst in which a noble metal such as platinum or rhodium is held by alumina or the like. A catalyst carrier which holds the catalyst described above is cylindrical in form, has numerous minute cells having honeycomb shaped cross section formed thereinside, and catalyst is coated on cell walls which form the cells. The three noxious ingredients mentioned above flow through the catalyst carrier, which reaches 200 to 300° C., in the exhaust gas, a chemical reaction occurs on the coated catalyst to render these noxious ingredients harmless, and thus exhaust gas can be cleaned.

[0005]FIG. 9 shows an example of a conventional catalytic converter having such a catalyst carrier. A catalyst carrier 1 is packed in a cylindrical casing 2, and a zonal mat 10 in which ceramic fiber is bound by a binder is rolled singly along a circumferential direction on the central part of the outer circumference of the catalyst carrier 1. This inmat 10 has heat resistance and cushioning performance because a catalyst carrier which has relatively low strength must be fixed in the casing. The mat 10 is packed in the casing 2 being compressed to hold the catalyst carrier 1 in the casing 2. Arrows show the flow direction of the exhaust gas in FIG. 9.

[0006] The mat 10 described above has a convex part 11 which is projecting in the rolling direction on one side of the rolling direction of the mat as shown in FIG. 10, and a concave part 12 is formed on the other side of the rolling direction of the mat. In a condition in which the mat 10 is rolled around the catalyst carrier 1, the convex part 11 is fitted into the concave part 12 and these edge parts are facing each other. By fitting the convex part 11 into the concave part 12 in this way, not only is the flow of exhaust gas through a clearance between these edge parts prevented by blocking the clearance with the convex part 11, the shifting of the edge parts in the axial direction is also prevented. Furthermore, this mat keeps the catalyst carrier hot. Such a mat is disclosed in Japanese Unexamined Patent Application Publication No. 217426/95. In this publication, clearance which exists at both sides of a convex part are extending at different positions along the rolling direction to prevent flowing out of fibers of a mat.

[0007] In a conventional mat 10 shown in FIG. 10, a convex part 11 and a concave part 12 extend parallel to the rolling direction so as to fit the convex part 11 and the concave part 12 to each other in the case in which a catalyst carrier having different a diameter is used, that is to say, so as to enable control of the rolling diameter. Therefore, each of the corner parts 11 a and 12 a included in the convex part 11 and concave part 12 are formed so as to be almost right-angled, and slightly R-shaped. In the case in which mat 10 is rolled around the catalyst carrier, rolling wrinkles which extend along the axial direction inevitably occur on the inner circumference of the mat 10, and such rolling wrinkles easily occur partially from the convex corner parts 11 a and 12 a as origins. FIG. 11 shows a condition in which a conventional mat such as mat 10 mentioned above is rolled around a catalyst carrier, and several rolling wrinkles partially occurred. These rolling wrinkles occur as the mat is being bent at a corner part as an origin.

[0008] If the catalyst carrier 1 and the mat 10 are canned, in other words, packed in the casing 2 as shown in FIG. 10 under the conditions in which rolling wrinkles partially occur as described above, surface pressure against the catalyst carrier 1 brought from around the rolling wrinkles on compressed mat 10 is increased, and the catalyst carrier may break in the case in which the surface pressure exceeds the breaking strength. Therefore, it is desirable that rolling wrinkles 13 occur equally along the circumferential direction to reduce the surface pressure. However, this is difficult because the corner parts 11 a and 12 a exist as described above.

[0009] Furthermore, in a process for production of such a catalytic converter, a technique in which a mat is compressed under a canning condition as mentioned above is required. As a conventional example of such a producing method, there is a method in which a catalyst carrier having a mat on its outer circumference is set inside of a pair of half members which compose a casing, and the half members are joined together. However, in this producing method, although the mat is compressed sufficiently along the joined direction of the half members, compression along the vertical direction of the joined direction is not sufficient. Therefore, there is a problem in that holding ability of the catalyst carrier cannot be stabilized. Therefore, a producing method in which a casing formed by rolling a flat plate was put on a catalyst carrier rolled by a mat, the diameter of the casing was shrunk by compressing the mat, and edges of the casing were welded, and thus the mat can be uniformly compressed along the whole circumference, was suggested.

[0010] Such a producing method is disclosed in Japanese Unexamined Patent Application Publication No. 291426/2000, and furthermore, in the publication, a technique in which surface pressure of a mat against a catalyst carrier is maintained constant by controlling the degree of diameter shrinking (called the “degree of tightening of outer shell” in the publication) of a casing depending on the outer shape of the catalyst carrier. In addition, as a producing method in which the diameter of a casing is reduced, there is also a tube shrinking method in which a cylindrically formed casing is shrunk. In this method, a process of welding each of the casings after reducing the diameter can be omitted.

[0011] A mat used in catalytic converters is obtained by applying a process of a kind of paper-production to ceramic fibers which mainly contain alumina-silica, or to heat resistant fibers such as alumina fibers or mullite fibers. Furthermore, it is common for vermiculite to be added to make it have a characteristic of expansion under high temperatures. There is also a mat in which vermiculite is not contained. Because the mat is prepared by such components and the producing process as described above, it is difficult to reduce reduction of surface density of the mat. Generally, it is said that there is a reduction in surface density of 8 to 10% at most by weight ratio. Because of such reduction in surface density, it is difficult to maintain surface pressure of a mat against a catalyst carrier at a constant value even if the diameter of the casing is reduced depending on the outer diameter of the a catalyst carrier.

[0012] In particular, a recent catalyst carrier shows a tendency to thin cell walls to increase the number of cells, so as to improve cleaning ability and to reduce exhaust resistance by increasing through cross section of exhaust gas. Therefore, breaking strength of the catalyst carrier is deteriorated compared to a conventional one. FIG. 12A shows an end face of a catalyst carrier of the thin wall type in which cleaning ability is improved, and such a catalyst carrier is usually produced by extrusion. In this extruding process, deformation easily occurs at a cell wall because of uniform flow of base material passing through a slit metallic mold for extrusion. In particular, this deformation easily occurs around the outer wall, and FIG. 12B shows an example of a catalyst carrier in which a cell wall near the outer wall is deformed. The strength may be further deteriorated because such a deformed cell wall can be easily buckled if compressed.

[0013] Therefore, in a recent thin wall type catalyst carrier, it is required that surface pressure which is brought by a mat under canning condition not further exceed the minimum required surface pressure to be held in the casing. To realize this, it is necessary to prevent the occurrence of partial rolling wrinkles originating from corner parts described above.

[0014] Furthermore, surface pressure against a catalyst carrier caused by a mat in a canning condition must be set between a pressure in which catalyst carrier does not shift if external force such as vibration or pressure of exhaust gas is exerted and a pressure which is lower than the breaking strength of the catalyst carrier. However, this set range of surface pressure becomes small because breaking surface pressure is decreased as breaking strength of catalyst carrier is decreased. Therefore, if dispersion of surface density of a mat is large as described above, surface pressure of the mat against the catalyst carrier easily deviates from the set range of surface pressures. In particular, in a tube shrinking method in which the diameter of a cylindrical casing is reduced, because the diameter of casing is slightly increased by spring back if the diameter-shrinking load is released, the degree of the diameter reduction must be decided by anticipating this phenomenon. However, it is difficult to control the degree of diameter reduction in which surface pressure is maintained constant in consideration of the spring back together with the deterioration of strength of a catalyst carrier.

SUMMARY OF THE INVENTION

[0015] Therefore, an object of the present invention is to provide a catalyst carrier holding mat in which partial increasing of surface pressure of the mat against a catalyst carrier under canning condition can be prevented by preventing partial occurrence of rolling wrinkles of the mat, and as a result, canning can be reliably applied even to a catalyst carrier having low strength without breaking. Furthermore, an object of the present invention is to provide a process for production of a catalytic converter in which under a tube shrinking method wherein a catalytic converter is produced by canning, surface pressure of a mat against a catalyst carrier can be controlled constantly for each pair of casing and catalyst carrier depending on reduction of surface density of the mat, and as a result, surface pressure of the mat can be set within a set range of surface pressure even in the case of a catalyst carrier having low strength.

[0016] Characteristics of the present invention are that in a catalyst carrier holding mat which is rolled around the outer circumference of a catalyst carrier along the circumferential direction, and holds the catalyst carrier in a casing by being compressed between the catalyst carrier and the casing, a convex part which is directed in the rolling direction is formed on one side of the rolling direction, a concave part in which the convex part extends parallel to the rolling direction so as to fit together to enable control of the rolling diameter, and a corner part of the convex part and the concave part are formed into R shapes of greater than 5 mm or a linear-chamfered shape.

[0017] In the present invention, bending of a mat originating from corner parts and rolling wrinkles are difficult to occur because corner parts included in a convex part and a concave part are not conventional right angles but are R shaped or linear-chamfered shapes. That is to say, partial occurring of rolling wrinkles originated from corner parts can be prevented. Therefore, under canning conditions, partial increase of surface pressure of a mat against a catalyst carrier brought by partially occurred rolling wrinkles can be prevented. As a result, canning can be reliably applied to even a catalyst carrier having low strength without breaking.

[0018] Furthermore, a process for production of a catalytic converter of the present invention is a method in which the inner diameter of a casing is calculated so as to maintain the filled density of a mat to be constant by regarding the outer diameter of a catalyst carrier which is a variable factor and surface density of the mat as parameters, diameter of the casing is reduced to obtain the calculated inner diameter of the casing, and a suitable degree of diameter shrinking of the casing under the diameter shrinking method is settled for each pair of the casing and the catalyst carrier. That is to say, characteristics of the present invention are that in a producing method of a catalytic converter in which a cylindrical catalyst carrier rolled by a mat is inserted into a cylindrical casing and the mat is compressed by reducing the diameter of the casing so as to have the catalyst carrier held in the casing, the inner diameter of the casing after diameter reduction is calculated by a fixed filled density of the mat after diameter reduction of the casing and surface density of a mat and outer diameter of a catalyst carrier of a pair of the catalyst carrier and the mat, and then the casing is shrunk until the inner diameter of the casing becomes the calculated inner diameter of the casing. As a casing of the present invention, a tube whose edges are welded beforehand, a seamless tube, or a cylindrically formed tube whose edge is not welded can be used. In the case of a tube whose edges are not welded beforehand, the edges are tacked temporarily if necessary after shrinking of diameter, and welded.

[0019] Inner diameter D (mm) of a casing of the present invention can be calculated by following formula (1). In the case in which the outer diameter of a catalyst carrier is X (mm), surface density of a mat is Y (g/m²), and filled density of a mat is GBD (g/cm³),

D=X+2Y/(GBD×10³)  (1)

[0020] In the present invention, the diameter of a casing is reduced until the diameter becomes the calculated diameter of the casing as described above, the calculated degree of diameter shrinking can keep filled density of the mat constant with high accuracy by settling the value of dispersion of plate thickness of the casing before diameter reduction and the value of changing of the plate thickness by diameter shrinking as corrective parameters.

[0021] In the case in which plate thickness is set as a corrective parameter is explained as follows.

[0022] Because a method in which a jig or the like which is put on an outer circumference of a casing are pushed to the inner direction of diameter is conducted to reduce the diameter of a casing, for example, the inner diameter of the casing is actually controlled by the inner surface of the jig which is contacted to the outer circumference of the casing, that is to say, the outer diameter of the casing. Inner diameter D (mm) of the casing can be calculated from the outer diameter Dout (mm) of the casing and plate thickness T (mm) of the casing by applying the following formula.

D=Dout−2T

[0023] Simultaneous equations of this formula and formula (1) are solved about Dout, and formula (2) can be obtained as follows.

Dout=2T+X+2Y/(GBD×10³)  (2)

[0024] By measuring the plate thickness beforehand before diameter reduction of the casing and applying the formula (2), filled density of a mat can be maintained constant at higher accuracy than in the case in which formula (1) is applied.

[0025] Furthermore, in the case in which changing of plate thickness is set as a corrective parameter is explained as follows.

[0026] If a tube whose edges are welded beforehand before diameter reduction or a seamless tube is used as a cylindrical casing, length and plate thickness of the shrunk casing is generally larger compared to the casing before diameter reduction. The reason for this phenomenon is that even if the diameter is shrunk, plastic deformation of lengthening and thickening occur to conserve the volume. Therefore, by adding the predicted value of plate thickness after diameter shrinking Tfrc as a corrective parameter to plate thickness T which was measured before diameter shrinking, surface density of the mat can be maintained constant at a high accuracy. Because plate thickness is increased by a conservation of volume, plate thickness after diameter reduction is increased as value of D or Dout is decreased if the diameter of the casing before diameter reduction is constant. This is expressed in a formula as follows.

Tfrc−T=f(D) or f(Dout)

[0027] An experiment in which D or Dout was intentionally changed to measure Tfrc and T was performed, a relationship between Tfrc-T and D or Dout was solved, and f can be obtained as a regression formula. If simultaneous equations of the relational expression of Tfrc and D or Dout obtained in this way and the formula (1) or (2) are solved about D or Dout, increasing of plate thickness by diameter reduction is reflected and the filled density of the mat can be maintained constant at a higher accuracy.

[0028] Next, an embodiment of the present invention is explained by way of FIGS. 1 and 2.

[0029]FIG. 1 shows a catalytic converter in which a mat of the embodiment is used. Reference numeral 20 is the mat, reference numerals 1 and 2 are a catalyst carrier and a casing similar to the ones shown in FIG. 9, respectively, and arrows show flow direction of exhaust gas. The mat 20 is a zonal object in which ceramic fiber is bound by a binder. This mat is singly rolled around the central part of outer circumference of the catalyst carrier 1 along the circumferential direction. The catalyst carrier 1 is canned in the casing 2 with the mat 20 rolled therearound. The mat 20 is compressed in this canning condition, and the catalyst carrier 1 is held in the casing 2 by being brought into contact with the mat at a fixed pressure.

[0030] As shown in FIG. 2, a convex part 21 which is directed in the rolling direction (longitudinally) is formed on an edge of the rolling direction of the mat 20, and a concave part 22 which is fitting to the convex part 21 is formed on the other edge of the rolling direction of the mat 20. The convex part 21 and the concave part 22 are formed on the center of the width direction of the mat 20, and each width is set to be one-third of the width of the mat 20. The width of mat 20 can be suitably selected depending on the length of the axial direction of the catalyst carrier 1, and is usually about 45 to 120 mm. The length of mat 20 is set as a suitable length relative to the circumference of the catalyst carrier 1. The convex part 21 and the concave part 22 are extending parallel to the rolling direction so as to fit each other even in the case in which the diameter of a catalyst carrier 1 is different, that is to say, so as to enable control of a rolling diameter.

[0031] Concave parts 21 a existing at both sides of the convex part 21, between the convex part 21 and a principal part 20A (a part in which edge parts of the convex part 21 and the concave part 22 are removed) of the mat 20, are formed into a quarter arc shape having a radius R of 5 mm or more, respectively. Furthermore, convex corner parts 21 b existing on the tip of the convex part 21 are also formed into the quarter arc shape having an R of 5 mm or more. On the other hand, concave corner parts 22 a existing at both side of the concave part 22, between the back tip of the concave part 22 and the principal part 20A of the mat 20, are formed into the quarter arc shape having an R or 5 mm or more. Furthermore, convex corner parts 22 b existing on the concave part 22-side of tips of a pair of edge parts 22A on both sides of the concave part 22 are also formed into the quarter arc shape having an R of 5 mm or more.

[0032] The mat 20 mentioned above is rolled around the outer circumference of the catalyst carrier 1 along the circumferential direction, the convex part 21 is fitted into the concave part 22 while maintaining a condition of these edges facing each other as shown in FIG. 2, and the mat 20 is canned in the casing 2 with the catalyst carrier 1 as shown in FIG. 1. According to a catalytic converter shown in FIG. 1, exhaust gas flows through numerous cells in the catalyst carrier 1 under conditions in which the catalyst carrier 1 is heated to more than activation temperature by exhaust gas, noxious ingredients in the exhaust gas are rendered harmless by chemical reaction, and thus exhaust gas is cleaned. Because the convex part 21 is fitting to concave part 22, clearance between the edge parts of the mat 20 is blocked to prevent exhaust gas from flowing in the clearance, and furthermore, the edge part does not shift in the axial direction.

[0033] In the mat 20 of the embodiment, each concave corner parts 21 a and 22 a which is included in the convex part 21 and the concave part 22 are formed into semi-circular arc shapes having an R of 5 mm or more. Although these corner parts 21 a and 22 a cause origins of rolling wrinkles in the condition in which the mat is rolled around the catalyst carrier 1 if they are formed into right angles as conventionally, the rolling wrinkles which are originated from the corner parts 21 a and 22 a are difficult to occur if the corner parts are formed into an R shape. That is to say, partial occurring of rolling wrinkles which are originated from the corner parts 21 a and 22 a can be prevented. Therefore, in the canning condition, partial increasing of surface pressure of the mat 20 against the catalyst carrier 1 by partial rolling wrinkles can be prevented, and as a result, canning can be certainly performed without breaking even if strength of the catalyst carrier 1 is low.

[0034] In other words, canning can be performed by using the mat 20 of the embodiment even if a wall which is forming cells of the catalyst carrier 1 is thinned and its strength is reduced. Therefore, engine output can be improved as pressure loss in the passing of exhaust gas is reduced, and the catalyst carrier 1 can be quickly activated as heat capacity is decreased to enable reducing amount of exhaust gas at engine starting which is.containing noxious ingredients.

[0035] Next, another embodiment of the present invention is explained by way of FIG. 3.

[0036] In a mat 30 shown in FIG. 3, each of the corner parts 21 a, 21 b, 22 a, and 22 b which are formed into semicircular arc shapes in the embodiment mentioned above are formed into linear-chamfered shapes by 5 mm or more. The linear-chamfered shape is a shape whose two edges forming the corner part are connected by line to fill in the corner in the case of concave corner parts 21 a and 22 a, and a shape whose two edges forming the corner are connected by a line to cut off the corner in the case of convex corner parts 21 b and 22 b. Furthermore, the size of 5 mm is equivalent to the filled part of concave corner parts 21 a and 22 a, or length of a base of an isosceles triangle removed from concave corner parts 21 b and 22 b as shown L in FIG. 3.

[0037] Also, in another embodiment shown in FIG. 3, similar to the embodiment mentioned above, partial occurring of rolling wrinkles which are originated from the concave corner parts 21 a and 22 a can be prevented when the mat 30 is rolled around the catalyst carrier 1, partial increasing of surface pressure of the mat 30 against the catalyst carrier 1 by the rolling wrinkles can be prevented under canning condition, and as a result, even a catalyst carrier having low strength can be certainly canned without breaking.

[0038] In addition, it is more desirable that plural grooves extending in width direction be formed beforehand on the inner circumference of the mat 20 at a constant interval to make rolling wrinkles occur uniformly in the circumferential direction, so as to prevent partial occurring of rolling wrinkles. FIG. 4 shows the inside of such a processed catalyst carrier.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0039]FIG. 1 is a drawing showing a cross section of a catalytic converter applied by a mat of an embodiment of the present invention.

[0040]FIG. 2 is a plane drawing showing both edges of a mat of an embodiment of the present invention.

[0041]FIG. 3 is a plane drawing showing both edges of a mat of another embodiment of the present invention.

[0042]FIG. 4 is a photograph showing the inside of a mat of another embodiment of the present invention.

[0043]FIG. 5 is a graph showing the result of measurement of breaking strength of the catalyst carrier (thick wall type).

[0044]FIG. 6 is a graph showing the result of measurement of breaking strength of the catalyst carrier (thin wall type).

[0045]FIG. 7 is a photograph showing the result of measurement of surface pressure distribution of a mat of an Example.

[0046]FIG. 8 is a photograph showing the result of measurement of surface pressure distribution of a mat of a Comparative Example.

[0047]FIG. 9 is a drawing showing a cross section of a conventional catalytic converter.

[0048]FIG. 10 is a plane drawing showing both edges of a mat of a conventional catalytic converter.

[0049]FIG. 11 is a photograph showing a cross section of a conventional catalytic converter.

[0050]FIG. 12A is a photograph showing an edge of a catalytic converter of the thin type, and FIG. 12B is a photograph showing an edge of a catalytic converter of the thin type whose cell walls are deformed.

[0051]FIG. 13A is a drawing showing a side cross section of a tube shrinking device used in an Example, and FIG. 13B is a cross section B-B line arrow along in FIG. 13A.

[0052]FIG. 14 is a graph showing the relationship of filled density of mat and surface pressure measured in an Example.

[0053]FIG. 15 is a graph showing the relationship of surface density of mat and surface pressure of mat measured in an Example.

EXAMPLES (1) Test for occurrence of rolling wrinkles

[0054] Four kinds of mats of the same shape as in FIG. 2 having dimensions of width 60 mm, length 365 mm, and thickness 11 mm, whose widths of convex parts and concave parts were 20 mm, whose lengths of convex parts and concave parts were 65 mm, and whose R (radius) of each corner parts were 5 mm, 7 mm, 10 mm and 15 mm, were prepared as Example 1 to 4. On the other hand, a mat in which the R of each corner part was 2 mm was prepared as a Comparative Example. These mats of the Examples and Comparative Example were rolled around a catalyst carrier having a diameter of 106 mm and the convex parts and concave parts were joined together. This rolling operation was repeated 5 times for each mat, and the occurrence of rolling wrinkles at corner parts were observed visually in each operation. Results are shown in Table 1. TABLE 1 R size Test number (mm) 1 2 3 4 5 Comparative 2 Occurred Occurred Occurred Occurred Occurred Example Example 1 5 Occurred Did not occur Occurred Did not occur Did not occur Example 2 7 Did not occur Did not occur Did not occur Did not occur Did not occur Example 3 10 Did not occur Did not occur Did not occur Did not occur Did not occur Example 4 15 Did not occur Did not occur Did not occur Did not occur Did not occur

[0055] As shown in Table 1, occurring of rolling wrinkles can be certainly prevented when the radius of corner parts are 7 mm or more, and there were cases in which occurrence of rolling wrinkles were prevented even in the case in which the radius of corner parts were 5 mm. Rolling wrinkles certainly occurred in the case in which the radius of corner parts were 2 mm. Therefore, it was confirmed that the occurrence of rolling wrinkles can be prevented in the case in which the radius of corner parts were 5 mm or more.

(2) Evaluation of effects of breaking of a catalyst carrier due to increase of surface pressure due to occurrence of rolling wrinkles

[0056] Breaking strength of two kinds of catalyst carriers in which thickness of walls forming each cell was 110 ì m and 65 ì m were tested by compressing. The catalyst carrier of the thick wall type having a wall thickness of 110 1ì m was a conventional product whose cleaning ability was relatively low, and the number of samples used in the breaking strength test was 497. On the other hand, the catalyst carrier of the thin wall type having a wall thickness of 65 ì m was a recent product whose cleaning ability was higher than that of the conventional product, and the number of samples applied to the breaking strength test was 720. The results of the tests of the thick wall type are shown in FIG. 5, and the results of the tests of the thin wall type are shown in FIG. 6. As shown in FIG. 5, the catalyst carrier of the thick wall type was not broken until a pressure of 20 kgf/cm², and the average breaking strength was 74.9 kgf/cm². On the other hand, as shown in FIG. 6, the catalyst carrier of the thin wall type was not broken until a pressure of 6 kgf/cm², and the average breaking strength was 23.2 kgf/cm².

[0057] Next, a process in which a sheet shaped tactile sensor of a surface pressure distribution measuring device (produce by the Unitta Company) was rolled around a catalyst carrier before a mat was rolled therearound, it was put into a cylindrical casing, and canning (reduction of diameter of casing) was performed, was applied to the mat of Example 3 mentioned above whose R of the corner parts was 10 mm and to the mat of the Comparative Example mentioned above whose R of the corner parts was 2 mm. In these two cases, pressure of diameter reduction was the same. FIGS. 7 and 8 show the measured points including the corner parts of the mats of Example 3 and the Comparative Example, and show the surface pressure distribution data measured by the tactile sensors of the measured points. If these figures are compared, although the surface pressure of the mat against the catalyst carrier was in a range of 5.5 to 6.0 kgf/cm² at the most, surface pressure of the mat of the Comparative Example was in a range of 7.5 to 8.0 kgf/cm² at the most. Therefore, the mat of the Comparative Example cannot be used for the thin wall type catalyst carrier which exhibits the breaking strength shown in FIG. 6. On the other hand, the mat of Example 3 can be used for the thin wall type catalyst because the surface pressure against the catalyst carrier was low.

(3A) Experiments for determining the relationship between filled density of a mat and surface pressure of a mat

[0058] First, the relationship between the filled density of a mat for a catalyst converter and surface pressure of this mat was determined beforehand by experiment. FIGS. 13A and 13B show a tube shrinking device used in the experiment. The structure of this tube shrinking device is as follows. A fixed ring 2 is fixed on one edge of the inside of a cylindrical housing 1, a shift ring 3 which can shift freely along axial direction is put on the other edge, and numerous pieces 4 are put inside the fixed ring 2 and the shift ring 3 along the circumferential direction at a constant interval. A casing 10 is inserted through these pieces 4, and by sliding the shift ring 3 toward the fixed ring 2 to compress the pieces 4, each piece is shifted to the inside of the diameter direction, and as a result, the diameter of the casing 10 is reduced by compression of the pieces 4. In FIG. 13, reference numeral 11 indicates a catalyst carrier, and reference numeral 12 indicates a mat. The center of the axial direction of the outside surface of the pieces 4 is thickest, and the outside surface is inclined in a tapered shape from the central part toward both ends. The inner surface of the fixed ring and the shift ring are also formed in a tapered shape so as to come into contact with the outside of the pieces. The degree of diameter reduction of the casing can be controlled by the degree of sliding of the shift ring. In this case, the gradient of tapering of the shift ring is 5:1; therefore, the degree of sliding of the shift ring is five times as much as the degree of diameter reduction.

[0059] In the experiment, desired values of filled density of the mat were determined to be 0.500 g/cm³, 0.450 g/cm³, 0.400 g/cm³, and 0.350 g/cm³ as shown in Table 2, and three values of the surface pressure of the mat as experiment data for each desired value of filled density were measured, and thus twelve values of surface density were measured in total (Experiments No. 1 to 12). As shown in Table 2, three kinds of catalyst carrier having different outer diameters corresponded to each filled density, and furthermore, the surface density of the mat which was put together with each catalyst carrier. It should be noted that the outer diameter of the catalyst carrier was measured by calipers or laser dimension measuring apparatus, and that surface density of the mat was calculated by measuring the weight of the mat with electronic force balance, the measured weight was divided by the area of the mat. TABLE 2 Experiment No. 1 2 3 4 5 6 7 8 9 10 11 12 Filled density of mat 0.500 0.500 0.500 0.450 0.450 0.450 0.400 0.400 0.400 0.350 0.350 0.350 (g/cm³) Outer diameter of 105.71 105.43 105.55 105.71 105.43 105.55 105.71 105.43 105.55 105.71 105.43 105.55 catalyst carrier: X (mm) Surface density of mat: 2682 2388 2454 2662 2575 2510 2647 2629 2485 2700 2502 2605 Y (g/m²) Degree of sliding of 31.29 38.56 36.64 25.77 29.11 29.95 18.75 20.62 23.60 7.78 14.86 11.29 shift ring (mm) Surface pressure of mat 3.61 4.88 4.05 2.38 3.06 3.27 1.29 1.56 2.02 0.69 1.12 0.64 (kgf/cm²)

[0060] Next, the degree of diameter reduction of the casing in which the determined filled density of the mat can be found was calculated by the formula of degree of diameter reduction (3) as follows. The results are shown in Table 2.

Z=a+bX+cY  (3)

[0061] (Z: Degree of diameter reduction of a casing (mm); X: Outer diameter of a catalyst carrier (mm); Y: Surface density of mat (g/m²); a, b, c: constants)

[0062] The formula mentioned above can be determined as follows.

[0063] First, correlation between filled density and surface pressure of the mat was determined by experiments, and surface density of the mat of desired value was calculated from the results as follows.

[0064] GBD: Filled density of the mat of desired value

[0065] Y: Surface density of the mat (g/cm²)

[0066] X: Outer diameter of the catalyst carrier (mm)

[0067] Z: Degree of reduced diameter of the casing (mm)

[0068] Db: Inner diameter of the casing before compression (mm)

[0069] Da: Inner diameter of the casing after compression (mm) Each of GBD to Da are defined as above, and two formulas can be established as follows.

GBD=2Y/(Da−X)

Z=Db−Da

[0070] Simultaneous equations of these two formulas are solved by substituting actual numerical values into GBD and Db which are constants, and constant terms a, b, and c in the formula (3) mentioned above can be obtained.

[0071] Next, as shown in Table 2, the degree of sliding of the shift ring in the tube shrinking device was calculated by multiplying calculated degree of diameter shrinking in each of experiment Nos. 1 to 12 by 5 times. Furthermore, in each experiment Nos. 1 to 12, a tactile sensor of surface pressure distribution measuring device (produced by the Unitta Company) was rolled around the catalyst carrier before the mat was rolled therearound, it was put into a casing prepared separately, and this casing was put into the tube shrinking device shown in FIG. 13. Next, the shift ring was shifted for calculated degree of sliding, and thus the diameter of the casing was reduced to perform canning. At the time, data of surface pressure distribution which were output from the tactile sensors were analyzed, and the average value as the surface pressure of the catalyst carrier brought by the mat was calculated. These surface pressure data are shown in Table 2, and a relationship between filled density of the mat and surface pressure is described in a graph shown in FIG. 14. The relationship between filled density of the mat and surface pressure became clear by this graph.

(3B) Canning test

[0072] Next, to obtain surface pressure of the mat of desired value, filled density of the mat which can obtain the surface pressure was decided based on FIG. 14. Inner diameter of the casing to be reduced was calculated from this filled density, measured outer diameter of the catalyst carrier, and surface density of the mat by the formula (1) mentioned above, and canning test in which the diameter of the casing was reduced was performed based on this. Specifically, a desired value of the surface pressure of the mat was set at 2.6 kgf/cm², and the filled density of the mat was calculated as 0.44 g/cm³ from FIG. 14. Canning tests were performed in the same manner as the experiment described above (3A), in which tactile sensors of a surface pressure distribution measuring device (produced by the Unitta Company) was rolled around the catalyst carrier and then the mat was rolled therearound, and the average of surface pressure on the catalyst carrier caused by the mat was calculated after canning. Number of specimens of the Examples was 18 (Specimens No. 1 to 18), and these data are shown in Table 3. On the other hand, as the Comparative Examples, canning tests in which the diameter of the casing was reduced so as to maintain the inner diameter constant were performed. Number of specimens was 5 (Specimens Nos. 19 to 23), and these data are also shown in Table 3. Furthermore, based on Table 3, the relationship between surface density of the mat and surface pressure of the mat after diameter reduction in each of the Examples and the Comparative Examples are described in a graph shown in FIG. 15. TABLE 3 Desired surface Outer diameter of Surface Inner Filled Surface Difference of Specimen pressure of mat catalyst carrier density of casing density of pressure surface pressure No. (kgf/cm²) (mm) mat (g/m²) (mm) mat (g/cm³) (kgf/cm³) (kgf/cm²) Examples 1 2.6 105.37 2417 116.36 0.44 3.15 0.56 2 2.6 105.60 2548 117.19 0.44 2.92 0.33 3 2.6 105.58 2595 117.38 0.44 2.33 0.25 4 2.6 105.61 2460 116.80 0.44 3.31 0.72 5 2.6 105.58 2582 117.32 0.44 2.57 0.02 6 2.6 105.62 2446 116.75 0.44 3.04 0.46 7 2.6 105.58 2649 117.62 0.44 2.60 0.02 8 2.6 105.63 2546 117.21 0.44 2.73 0.14 9 2.6 105.60 2401 116.52 0.44 2.61 0.03 10 2.6 105.63 2440 116.72 0.44 2.98 0.40 11 2.6 106.29 2662 118.39 0.44 2.50 0.08 12 2.6 106.29 2605 118.14 0.44 2.42 0.16 13 2.6 106.34 2656 118.41 0.44 2.47 0.11 14 2.6 106.33 2624 118.27 0.44 2.48 0.10 15 2.6 104.83 2347 115.50 0.44 2.71 0.13 16 2.6 104.93 2370 115.70 0.44 3.17 0.59 17 2.6 104.94 2433 116.00 0.44 2.84 0.26 18 2.6 104.89 2382 115.72 0.44 2.91 0.33 Average 2.76 0.18 ó 0.288 0.288 Comparative 19 2.6 106.33 2688 117.06 0.501 4.24 1.65 Examples 20 2.6 104.83 2330 117.06 0.381 1.32 1.26 21 2.6 105.60 2541 117.06 0.443 2.67 0.08 22 2.6 105.58 2496 117.06 0.435 2.46 0.12 23 2.6 105.63 2466 117.06 0.431 2.38 0.20 Average 2.55 0.03 ó 0.783 1.046

[0073] According to Table 3, the average value of surface pressure of the mat is relatively close to the desired value in both the Examples and the Comparative Examples. However, large variations in the surface density of the mat were observed in the Comparative Examples, and the differences of surface pressure (desired surface pressure of mat) was about 3.6 times as great as in the Examples. Therefore, it was confirmed that even if there was variation in surface density of a mat or outer diameter of a catalyst carrier, the surface pressure can be controlled to be constant depending on the variation in the process for production of the present invention. In addition, because surface pressure of a mat can be controlled as described above, breaking of catalyst carrier can be prevented by controlling the surface pressure of the mat within a set range even in the case of a catalyst carrier having low breaking strength. Furthermore, as is obvious from FIG. 15, it is confirmed that there is a correlation between surface density of a mat and surface pressure of a mat after the diameter was reduced. Therefore, in a mat which is compressed in a constant filled density, surface pressure changes occurred correlating with surface density before the mat was compressed. Therefore, the surface pressure can be stabilized further by calculating the filled density which is corrected depending on the surface density of the mat before compression by making this correlation as an index, and then reducing the diameter of the casing.

[0074] As explained above, in the catalyst carrier holding mat of the present invention, partial occurrence of rolling wrinkles can be prevented and partial increasing of surface pressure of the mat against the catalyst carrier under canning condition can be also prevented by this because corner parts included in convex part and concave part fitted to each other when rolled around the catalyst carrier are formed into an R shape having a radius of 5 mm or more, or formed into a linear-chamfered shape, and as a result, even a catalyst carrier having low strength can be reliably canned without breaking.

[0075] Furthermore, in the process for production of the catalytic converter of the present invention, the filled density of a mat having correlation with the surface pressure of the mat against a catalyst carrier is calculated for each catalytic converter which is produced, and the degree of diameter reduction of the casing is controlled depending on this, and a catalytic converter in which surface pressure of the mat against the catalyst carrier is constant can be produced. 

What is claimed is:
 1. A catalyst carrier holding mat rolled around an outer surface of a catalyst carrier accommodated in a cylindrical casing along a circumferential rolling direction, the mat holding the catalyst carrier in the casing by being compressed between the catalyst carrier and the casing, the mat comprising: a convex part formed on one side of the rolling direction of the mat and projecting in the rolling direction, a concave part formed on the other side of the mat to fit with the convex part, the convex part and the concave part extending parallel to the rolling direction in condition of fitting together to enable control of the rolling diameter, and corner parts included in the convex part and the concave part, the corner part formed into an R shape with a radius of not less than 5 mm or a linear-chamfered shape.
 2. A producing method for a catalytic converter comprising the following steps: inserting a cylindrical catalyst carrier rolled by a mat into a cylindrical casing, compressing the mat by reducing the diameter of the casing so as to hold the catalyst carrier in the casing, wherein calculating an inner diameter of the casing after reducing diameter thereof based on a surface density of the mat and an outer diameter of the catalyst carrier before the reducing diameter and a predetermined filled density of the mat after reducing diameter of these mat and catalyst carrier, and reducing the casing until an inner diameter of the casing reaches the calculated inner diameter of the casing. 